(1) Field of Invention
The present invention relates to an expandable stent and, more particularly, to an expandable stent that collapses into a non-convex shape and expands into a convex shape.
(2) Description of Related Art
Stents have been devised for a variety of purposes. For example, stents are often formed as coronary stents or used in cardiac valve prostheses for implantation into the human body. Previously, the large diameter and bulky structure of common prosthetic valves required implementation via open heart surgeries. During the past few years, percutaneous heart valve (PHV) replacement and repair has emerged as an additional therapy to potentially avoid the re-operation in young patients with congenital heart disease or severely sick patients. The challenge in PHV technology is to develop a collapsible and durable PHV that can be delivered through a reasonably small diameter lumen catheter, non-interfering with the normal anatomy, and being competent without significant valve stenosis.
For example, a fully formed valve which is deliverable via a catheter is disclosed in U.S. Pat. No. 6,582,462 to Andersen et al. (the '462 patent), the entire contents of which are incorporated herein by reference. The '462 patent discloses a fully formed valve prosthesis comprising a collapsible elastic valve mounted on an elastic stent. The commissural points of the elastic collapsible valve are mounted on the cylindrical surface of the elastic stent. A catheter technique permits delivery of the fully formed valve to the target area without the need for surgical intervention in the body. Once at the target site, the collapsed and fully-formed valve and elastic stent are expanded. A disadvantage to this design is the relatively large size of the compressed radial diameter of the fully formed valve and corresponding stent when delivered to the target area.
Another example is U.S. Pat. No. 6,530,952 to Vesely (the '952 patent), the entire contents of which are incorporated herein by reference. The '952 patent discloses a cardiovascular valve system including a permanent base unit that is affixed to the patient using conventional sutures or staples, and a collapsible valve having a collapsible frame that mates with the permanent base unit, and supports valve leaflets. An installed collapsible frame may be re-collapsed and disengaged from the permanent housing whereas a new collapsible valve is then installed, to resume the function of the prosthesis. As was the case above, a drawback to the device of the '952 patent is that, although collapsed, the valve and corresponding stent have a relatively large, radial diameter.
Another example can be found in U.S. Pat. No. 7,803,168 to Gifford (the '168 patent), the entire contents of which are incorporated herein by reference. The '168 patent teaches a device for decalcifying an aortic valve. The device breaks up calcific deposits in and around the aortic valve through application or removal of heat energy from the calcific deposits. In doing so, the device includes an implantable structure with a balloon. To expand to the structure, the balloon is inflated. Again, the structure includes a relatively large, radial diameter which can create some difficulties in some patients.
The examples provided above illustrate various devices that expand upon arrival at the expansion point within the heart or other desired location. While operable for percutaneous insertion or other uses, the examples require stents having a relatively large, radial diameter.
Another example can be found in U.S. Pat. No. 7,175,656 to Khairkhahan (the '656 patent), the entire contents of which are incorporated herein by reference. The '656 patent teaches a different form of a valve replacement in which additional stent deployment mechanisms are obviated. As described in the '656 patent, the stent mechanism can be formed of a framework of resilient metals, such as superelastic shape memory alloys. Thus, once deployed to the appropriate location, the material of the stent itself causes its expansion into the requisite shape. While the '656 patent teaches use of a superelastic shape memory alloy, it still requires a stent with a relatively large, radial diameter.
Each of the prior art designs has certain disadvantages. However, they all share the common disadvantage in that the stents possess a radial diameter that is undesirable.
Thus, a continuing need exists for a deployable stent which has a minimal radial profile to increase the efficacy of reaching the desired deployment location and minimize the risk to a patient.